LDEV size calculations

Provisioning Guide for VSP One Block

Version
10.2.x
Audience
anonymous
Part Number
MK-23VSP1B012-00

The capacity of a volume created from a DDP group cannot be specified for each volume. For this reason, the capacity calculations described in this topic are not required.

When you create custom-sized volumes (CVs), you can specify the capacity of each CV. However, rounding will produce different values for the user-specified CV capacity and the actual entire CV capacity. To calculate the actual capacity of a CV, you must use a mathematical formula. These topics explain how to calculate the user area capacity and the entire capacity of a CV.

The entire capacity of a CV or an LDEV consists of the sum of two types of capacity. One type is the user area capacity that stores the user data. The second type is the capacity of all areas that are necessary for an LDEV implementation including control information.

The CV capacity that the host recognizes is the same as the CV capacity calculated by the formula. If the LDEV capacity unit is set to block by using the raidcom add ldev command, the LDEV capacity calculated by using the formula is set. However, if the LDEV capacity unit is set to MB, GB, or TB by using the raidcom add ldev command, an error might occur in the LDEV capacity due to the unit conversion. To set the correct LDEV capacity, set the LDEV capacity unit to block.

Implemented LDEVs consume the entire capacity from the external volume group capacity. Therefore, even if the sum of user areas of multiple CVs is the same as the user area of one CV, the remaining free space generated when multiple CVs are created can be smaller than the free space in the DDP group when one CV is created.

To efficiently use the external volume group capacity, make sure to create an LDEV so that the total capacity of a CV is not corrected. Calculate the entire capacity based on the user area capacity of the CV by referring to Calculation of an OPEN-V volume size (CV capacity unit is MB) or Calculation of an OPEN-V volume size (CV capacity unit is blocks). The LDEV capacity specified by using the raidcom add ldev command might be corrected. Set the capacity by referring to Calculation of an OPEN-V volume size (CV capacity unit is blocks).

If the available volume capacity of each free space (block) is not divisible by (boundary-value x 2), all available volume capacity cannot be allocated to LDEVs. The remainder from the division of the available capacity by (boundary-value x 2) is left. In addition, all available capacity might not be used depending on the external volume group capacity and the RAID level.

When you create an LDEV with the raidcom add ldev command, the capacity of the LDEV created depends on whether offset_capacity option is specified, even if the specified capacity value is the same.

  • If the -offset_capacity option is specified, the capacity value entered is corrected to match the boundary value. In the case of a boundary corrected LDEV, the capacity of the LDEV is compensated to the unit that manages the data, which improves drive utilization.
  • If you do not specify the -offset_capacity option, an LDEV is created with the capacity value entered.

Within the storage system, data is managed on a per-slot basis, and that data is protected in DDP stripe units.